A 61-year-old man with periodontitis and dental caries who complained of fever and chest pain was diagnosed at another hospital as having pneumonia. He improved over 10 days with administration of antibiotics. Afterward, he was asymptomatic for the next one and one-half months, but then fever, chest pain, and cough developed again. He was readmitted to the same hospital and diagnosed as having pneumonia, and antibiotics were administered for 4 days. However, infiltration expanded, and on the 5th hospital day he was transferred to our hospital, where he was diagnosed as having lung abscess and empyema. Aspiration of pleural fluid showed frank pus, and Actinomyces meyeri and Peptostreptococcus anaerobius were isolated. Later, A. meyeri and Pe. anaerobius were correctly identified as Pseudoramibacter alactolyticus and Pe. stomatis, respectively, by 16S rRNA gene clone library analysis. The patient improved with the administration of antibiotics (ampicillin/sulbactam).